-- 插入字段金额
-- 删除字段金额
alter table users drop column balance;
-- 新增字段金额
alter table users add column balance decimal(10,2) not null default 0 comment '金额';
select * from users;
select username,balance from users;

-- 开始事务
begin ;
-- 转账
update users set balance = balance - 100 where username = '王五';
update users set balance = balance + 100 where username = '李四';
-- 提交事务
commit;
-- 回滚事务
# rollback;
